Hot Spot Stress Method Research Articles

Experimental and numerical investigations on stress concentration factors of concrete filled steel tube X-joints.

An SHS-CFSHS X-joint is fabricated by welding two square hollow section (SHS) braces to a concrete-filled square hollow section (CFSHS) chord. In this paper, the stress concentration factors (SCFs) of SHS-CFSHS X-joints are investigated through experimental tests and finite element analysis (FEA), with the hot spot stress method serving as the analytical approach. Eight specimens are designed and manufactured, with FE models built in software ANSYS. These FE models are validated against the test results. The specimens are tested under brace axial tension to determine the SCFs of the X-joints. It shows that the concrete filled in the chord effectively reduces the SCFs of the X-joints. To further explore various load conditions and the influence of the parameters, FEA is carried out and a total of 64 FE models are built. Based on the FEA results, multiple regression analysis is used to obtain the SCF formulae of SHS-CFSHS X-joints under axial tension load and in-plane bending load in the brace, respectively. Comparison and analysis of the SCF results obtained from experimental tests, the proposed formulae, and FE simulations reveal that the formulae presented in this study are both conservative and suitable for predicting SCFs.

Critical distance-based fatigue evaluation of welded joints in steel bridge under bending loading

Welded joints are prone to fatigue cracking under bending loading in steel bridges. However, certain design curves for these joints primarily focus on tensile or bending-tension loads, potentially leading to inaccurate assessments. This study conducted bending fatigue tests on fillet welded joints (FWJs), evaluating the suitability of specification methods for assessing their fatigue performance and attempting to determine the strength curves required for fatigue assessment. Furthermore, it validated the feasibility of the traditional theory of critical distance (TCD) in predicting the fatigue life of FWJs under bending loading. On this basis, a TCD assessment method incorporating the welding residual stress (WRS) was proposed. The results indicate that the fatigue design curve of the specifications provides conservative predictions for the fatigue performance of FWJs under bending loading, and FAT112 from Eurocode3 may be suitable for fatigue assessment using hot-spot stress and nominal stress methods. Furthermore, predicting fatigue life for FWJs under bending loading with traditional TCD is feasible. Compared to traditional TCD, the proposed method significantly improves prediction accuracy, with a minimum scatter factor of just 1.5. The proposed TCD not only considers the WRS effect but also reduce the need for numerous fatigue tests to establish the design curve for a specific welded joint.

Fatigue life evaluation of laser welded lap joints of dissimilar aluminum alloys

Abstract Constant amplitude fatigue tests were conducted on 6061/7075 dissimilar aluminum alloy laser welded lap specimens, as well as weld line cross-section hardness measurements. The fatigue test results show that the specimens exhibit multiple fracture modes that exit near the weld seam. The microhardness data on weld line cross-section from 7075 side to 6061 side display a sharp change and the softening phenomenon is serious. The hardness variation in heat affected zone of laser welding is very shallow, and its hardness is close to that of the base material. It was found that there are slag inclusions and pores in the weld seam when observing the fatigue fracture surface using SEM, and a small amount of secondary cracks were generated. However, stress concentration plays a dominant role in causing specimen fracture under fatigue loading, rather than welding defects. Defective specimens are found to have higher fatigue strength. The fatigue life prediction results obtained by the notch stress method and the hot spot stress method are both conservative and fall within two factor lines. The hot spot stress method has relatively higher accuracy for life prediction. The accuracy of both methods in predicting life is influenced by the location of the fracture.

Fatigue enhancement of welded thin-walled tubular joints made of lean duplex steel

In the present work, the fatigue strength and enhancement techniques of welded thin-walled (wall thickness of t = 2 mm) square hollow section (SHS) joints made of lean duplex stainless steel (grade EN 1.4162) are experimentally and numerically studied. Experimental fatigue tests were carried out on gas metal arc-welded tubular X-joints subjected to fully reversed (applied stress ratio of R = −1) constant amplitude in-plane bending moment. Geometrical improvement techniques were investigated by applying fully penetrating fillet welds and using structural reinforcements. The applications of high-frequency mechanical impact (HFMI) treatment were examined in the context of further enhancing the fatigue strength of the geometrically improved joints. In addition to the experimental work carried out in this study, the fatigue test data of welded thin-walled SHS and rectangular hollow section (RHS) joints were extracted from literature. Structural stress concentration factors were numerically and analytically obtained to validate the fatigue assessment model for welded thin-walled tubular joints using the structural hot-spot stress method. In addition, the effective notch stress (ENS) concept with the reference radius of rref = 0.05 mm was implemented. Compared to the joints in the as-welded state, fatigue strength improvements of 1.53–2.7 were found in the geometrically improved joints. On the other hand, the HFMI treatment did not lead to an additional increase in the fatigue strength capacity of the studied AW joints. The CIDECT guidelines for the structural hot-spot stress method were found conservative for thin-walled joints (t < 3 mm) and could be recommended for fatigue design.

Stress Concentration Factors of Concrete-Filled Double-Skin Tubular K-Joints

Tubular joints are important connecting parts of a welded steel tube structure. The S-N curves based on the hot spot stress (HSS) method are often used to evaluate the fatigue life of tubular joints in practical engineering. The stress concentration factor (SCF) is a key parameter to calculate HSS. In this paper, stress concentration tests of hollow-section and concrete-filled double-skin tubular (CFDST) K-joints were carried out, respectively, and then finite element models of K-joints considering the weld were established. The developed models were validated with the experimental results. The influence of key geometrical parameters, such as the diameter ratio of brace to chord β, the diameter to thickness ratio of chord γ, the wall thickness ratio of brace to chord τ, brace angle θ, and hollow section ratio ζ on the distribution and key position of SCFs along the weld toe, was discussed. Parametric studies were conducted to obtain the calculating equations for the SCF values of CFDST K-joints. The results demonstrate that infill concrete can effectively reduce SCFs along the weld on the chord. When the hollow section ratio was reduced to 0.317, the SCF was reduced by 77.2%. Notably, the SCF reduction rate was sensitive to γ and θ, with a decrease observed as γ increased. The hollow section ratio ζ had a less pronounced effect on SCF distribution patterns, but as ζ decreased, the chord’s stiffness improved, suggesting a potential approach to enhance joint performance. The distribution of SCFs is similar for joints of the same type but different geometric configurations. The innovatively integrated hollow section ratio in the CFDST design equation significantly simplifies and enhances the precision of SCF calculations for CFDST K-joints.

Fatigue Life and Crack Initiation of K and N Type Jacket Structure Using 3D Fatigue FE Analysis

Jacket structures are widely used as foundation structures for offshore wind energy. The jacket structure as a truss shape structure is not deformed easily and has the advantage of keeping the structure stable under wind load. The offshore jacket structures are subjected to unstable repetitive loads such as wind, and wave loads in the deep sea. These harsh environments lead to the deterioration of material performance and fatigue failure with cracks in the structure. It is crucial to locate fatigue cracks since they might compromise a structure's overall stability.In this study, fatigue life and crack initiation for different braced jacket structures were investigated. First, 3D non-steady heat conduction analysis and thermal elastic-plastic analysis were performed to reproduce the initial state of the weld joint in the jacket structure. The heat history obtained from the 3D non-steady heat conduction analysis was enforced as an input in thermal elastic-plastic analysis to calculate welding deformation and residual stress. Next, the fatigue FE analysis of the jacket welding structure was carried out using the residual stress and welding deformation as initial values along with the external cyclic loadings. The 3D fatigue FE analysis employed cyclic hysteresis constitutive equations and fatigue damage theory to calculate the fatigue life and crack initiation. The 3D fatigue FE results were compared with the S-N curve by European code 3 and the hot spot stress (HSS) method. The results show that the 3D fatigue FE method effectively calculates the fatigue life and finds crack initiation.

Master S-N Curve Fatigue Life Prediction of a Railroad Car Bogie Based on Mesh Insensitive Structural Stress Method

Fatigue life prediction of a welded structure is a complex phenomenon due to the nature of fatigue and the welding process. Additionally, Finite Element Method (FEM) results are extremely sensitive to the size of elements. Therefore, it is difficult to adopt a method to estimate the fatigue life, especially for welded structures. Besides, mesh size independence is a critical issue to perform fatigue life prediction methods that eliminates the need for excessive element numbers in the mesh. This paper investigates the Master S-N Curve Approach (MCA) using the output parameters of the mesh insensitive Structural Stress Method (SSM). MCA based on SSM employs structural stresses recovered from nodal forces and nodal moments. To recover these inputs, FEM model should be established properly. Thus, boundary conditions and applied loads were prepared for the model according to the BS EN 13749:2021. The submodeling technique in ANSYS software was used to analyze the bogie structure. To justify the mesh independence for the model, different mesh sizes were tested. In a specific range for shell bodies, SSM was shown to provide sufficient mesh independence feature. Furthermore, MCA was compared with Hot Spot Stress Method and Nominal Stress Method based on their fatigue life estimations.

A methodological approach to assessing the durability of welded structures of screens using SolidWorks Simulation software

Purpose. To develop methodological recommendations for studying the characteristics of screen welded assemblies of ГВЧ-31 С type using specialized tools of the SolidWorks Education Edition software package. Methodology. The methodology for analyzing welds when designing a vibrator fixation system is based on the use of current software tools, in particular SolidWorks Simulation in combination with SolidWorks Motion, as well as the HOT SPOT STRESS technology. To analyze the accuracy of determining the dynamic loads arising during the screen operation, the results of modeling using the SolidWorks Motion program are compared with those obtained by traditional methods using the Lagrange equation of the second kind. The computation discrepancy is within 10 %. Findings. It has been determined that it is rational to analyze welds using the SolidWorks Education Edition software shell, taking into account the least profitable option, namely, when their edges are not finished. Furthermore, during solid-state modeling, the formation of a cavity in the weld is simulated in the form of a gap between the welded elements. The stress-strain state analysis of welded constructions using the Finite Element Method has revealed that when the computational mesh cell size is reduced, the stress value increases significantly, which indicates the result singularity and the need for further application of the HOT SPOT STRESS method. In the course of research, it has been determined that the highest stresses in welds occur on the central rib. The duration of use of welded joints is more than 2·106 cycles. Originality. For the first time, the methodology has been substantiated for determining the service life of welded elements of screen constructions using the latest computer analysis tools. The method makes it possible to calculate the dynamic forces arising in constructions performing oscillatory movements, as well as to analyze the stress-strain state of welds using the HOT SPOT STRESS technology and assess their service life. Practical value. It has become possible to implement the method for calculating the stress-strain state of welds in the design of a vibrator fixation system and assessing their service life, which is important for analyzing and determining the parameters of various vibrating machines. The results derived from the investigation were incorporated by the local entity, Ukrprommineral, to modernize the screens.

Hot spot stress distribution and fatigue life evaluation of steel-UHPC deck in a long-span cable-stayed bridge

The steel-UHPC composite deck is an effective solution to fatigue cracks of orthotropic steel decks (OSD) in steel bridges. However, limited research has been reported on the hot spot stress distribution of the steel-UHPC composite deck. This study investigates the hot spot stress distribution and residual fatigue life evaluation of the steel-UHPC deck in a long-span cable-stayed bridge. The two extrapolation points' strain at fatigue-prone details were continuously measured under in-service traffic flow. The hot spot stress time history at each detail of the conventional OSD deck and steel-UHPC deck were analyzed and compared. The residual fatigue life of each detail was calculated. A three-dimensional finite element (FE) model was established and validated through experimental results. The bridge deck's deflection, the asphalt pavement's tensile stress, and the stress distribution at the floor beam cutout were numerically studied using the validated FE model. The results showed that the UHPC layer significantly reduced the hot spot stress and extended the fatigue life at each fatigue detail. The UHPC layer reduced the bridge deck's deflection and the asphalt pavement's tensile stress. Compared with nominal stress method, the hot spot stress method can better describe the large stress gradient distribution induced by the stress concentration at fatigue detail.

Fatigue performance assessment of thick TIG-Dressing cruciform welded joints made by Q355D structural steel

TIG-Dressing treatment is often used for steel welded joints in construction machinery manufacturing as an effective and reliable method for post-weld fatigue strength improvement. This paper investigates the fatigue performance of thick Q355D cruciform joints in heavy load-carrying steel structures under different TIG-Dressing treatments. Two TIG-Dressing treatments for the full-penetration welded joints were used for fatigue tests considering filling metal during this post-welded processing. Experimental tests studied the fatigue strength of TIG-Dressing cruciform welded joints of Q355D structural steel. The geometric parameters and relevant statistical analyses were performed by actual 3D optical measurement. Moreover, the stress concentration factors are calculated according to the actual geometrics of the welded joint. On the other hand, the effective notch stress and hot spot stress methods were employed to evaluate the fatigue data of Load-Carrying Welded joints (LCWJs) under different notch radius. The collected fatigue data from the literature was compared with the tested data by the above methods. The results show that the fatigue strength of LCWJs under TIG-Dressing processing without filler metal is higher than those under the same TIG-Dressing treatment with filler metal. The effective notch stress method with a fictitious notch radius r = rtrue + 1 mm (rtrue is the actual notch radius at weld toe) could obtain smaller scatter bands for the tested fatigue data.

Stress range in the welded cruciform joint in the shell model using the Hot Spot Stress Method

AbstractThis paper focuses on the linear extrapolation procedure for the Hot Spot Stress Method on a platés surface using shell models. Published papers have shown the benefits of solving the fatigue life by methodologies that integrate changes of the stress near the weld toe. This procedure is based on modelling the joints by solid elements. Current advanced connection models for steel structures are based on shell models which save computational time by an order of magnitude and allow the modelling of structural joints in a sufficiently short time using second‐order theory.Seven models for making shell welded connections are presented here, each of which was prepared for a specific joint. Many researchers have reported that non‐universality and deviations of the stress peak value in the first extrapolation point are major questions. A sensitivity study has indicated that there is a conditional stable ratio between the values of the stress ranges in the two evaluation points. A comparison between cases, that use experimental parameters and results taken from the literature, has shown that there is a reduction in the differences of the stress ranges between solid and shell models.

Fatigue behavior at flange thickness transitions with cope holes – design model for engineering practice

AbstractFlange thickness transitions are common constructional details in welded girders, such as crane runway girders or main girders of highway and railway bridges, to adapt the bending moment capacity of the girder to variable bending moment distributions. At these details the transverse butt weld is often decisive for the fatigue verification of the member. Based on comprehensive studies at Graz University of Technology, including also two fatigue tests, stress concentration factors for the fatigue verification of the butt welds at the girder flange were developed. However, due to the complex local stress fields at the flange thickness transition, cope holes were not included in these studies.Within a research cooperation of Graz University of Technology and Ruhr University Bochum, flange thickness transitions with cope holes are studied recently. Based on comprehensive numerical FE‐ calculations within the practically relevant geometric parameter range, the complex local stress fields near the cope hole are analyzed both, within the web and the flange plate. For validation of the FE‐model also strain measurements were carried out at large‐scale test girders. Applying the hot spot stress method, the decisive hot spots are determined, and a simplified design model is developed for an accurate prediction of these hot spot stresses.The paper presents this design model in detail and shows its accuracy ‐ compared to the FE‐results ‐ for a wide range of different geometrical conditions, as used in practice (e.g. variation of girder dimensions, flange thickness ratio, tapering to the outside and to the inside).Finally, also the results of full‐scale fatigue tests on two different welded girders with flange thickness transitions and cope holes are presented. This provides an accurate fatigue resistance for the proposed design model, based on the hot spot stress method.

Different fatigue design stress methods for common constructional details in cylindrical shells under axial loading – Influence of the r/t‐ratio and the shell thickness

AbstractCylindrical shells are frequently used in industrial constructions, for example in pipes, towers and masts and are often subjected to fatigue loading. Two common welded constructional details were studied: i) detail 1, the wall thickness transition with a circumferential butt weld and ii) detail 2, the welded ring stiffener. Both details are loaded with axial stresses due to N and/or M. In the new Eurocode prEN 1993‐1‐9:2021 three different fatigue design stress methods are provided in general. For these two constructional details, beside the modified nominal stress method, the hot spot stresses and effective notch stresses are determined with Finite‐Element‐Analyses, with variation of geometric parameters, like shell thickness t, radius‐over‐thickness‐ratio, t2/t1‐ratio at the thickness transition and thickness ts of the ring stiffener. Stress concentration factors for the different fatigue design stress methods are presented and the influence of the geometric parameters are investigated. Especially the difference between a plate with an infinite radius‐over‐thickness‐ratio and the pipe with a finite radius‐over‐thickness‐ratio is shown. Also, the influence of the shell thickness is investigated. For comparison of the three different methods the utilization factors of the hotspot stress method and the effective notch stress method are compared with those of the modified nominal stress method. A recommendation is given, which design stress method is appropriate for the two studied details, depending on the geometric parameters.

Study on Fatigue Cracking of Diaphragm's Arc Opening of OSD in Steel Bridges by Using Biaxial Stress Method.

Changes in loading position have a significant impact on the stress field of each vulnerable area of an orthotropic steel deck (OSD). The arc opening area of the diaphragm and the connecting area between the U-rib and the diaphragm under the moving load are prone to fatigue cracking. By comparing the stress responses under different methods, the hot spot stress (HSS) method is used as the main stress extraction method in fatigue performance evaluation. The control stress of fatigue cracking was analyzed by comparing the direction of the principal stress field with the crack direction in this experiment. According to the stress amplitude deviation under the biaxial stress state, a set of methods for evaluating the effects of in-plane biaxial fatigue was developed. An improved luffing fatigue assessment S-N curve was applied to analyze the fatigue life of the diaphragm's arc opening area. The results show that when the moving load is exactly above the connection of the deck and the web of the U-rib on one side, it is in the most unfavorable position in the transverse direction, and the diaphragm is mainly under the in-plane stress state. The longitudinal range of the stress influence line of the arc opening is approximately twice the diaphragm spacing. Two to three stress cycles are caused by one fatigue load. Fatigue crack control stress is the principal stress tangential to the arc opening's edge in this area. The normal direction of the principal stress in the model test is roughly consistent with the crack initiation direction. The variation in the stress amplitude deviation in this area is caused by changes in the action position of the moving load. When the moving load is at a certain distance from the involved diaphragm, it is reduced to zero, implying that the in-plane fatigue effect is the greatest in this area.

Experimental Study of Hot Spot Stress for Spatial CHS KK-Joints

In offshore structures such as offshore wind turbine jacket foundations, prolonged exposure to wind and wave loads can lead to fatigue failure, especially at the joint connections. Currently, international codes primarily evaluate the fatigue performance of tubular joints using the hot spot stress (HSS) method, with stress concentration factor (SCF) being the most crucial parameter. Numerous studies have investigated the SCF of tubular joints and proposed corresponding calculation formulas. However, most of these studies have focused on simple basic loading cases. To comprehensively understand the distribution pattern of the HSS method at spatial tubular joints, this study selects the spatial CHS KK-joints as the research subject and performs the HSS testing on spatial KK-joints. The research indicates that, in CHS joints, the distribution of the HSS on the chord side remains consistent under tensile or compressive loading conditions along the brace axis. Additionally, within spatial joints, the loading conditions of neighboring braces near the reference brace significantly affect the distribution of HSS on the chord side and exhibit varying spatial effects on the SCF. Furthermore, this study applies recommended calculation formulas from codes to analyze the experimental results. The findings reveal significant discrepancies between the SCF calculation formulas recommended by CIDECT and the test results, rendering it inadequate for accurately calculating the SCF of spatial KK-joints. Therefore, further research is required to develop suitable calculation formulas for SCFs in spatial KK-joints.

Comparison of Welded Joint Stress with Experimental and Finite Element Method Using of Hotspot Method

The purpose of the welded structures is to combine the two different structures defined as the workpiece and the main material in order to ensure that they remain in the elastic deformation zone by meeting the loading conditions safely. Welding parameters such as preheating, welding speed, shielding gas selection, filler wire selection, voltage, current values affect the mechanical properties of the HAZ zone and the general structure, especially when welding fine-grained structural steels are performed. Strain gauge sensors can give normal stress and shear stress values for structures forced by static loadings depending on the stable x, y, z axes. The hotspot stress method used with the finite element method gives closer results to experimental studies. In this study, two different S960QL steels were combined with workpiece and main material using MAG welding. Data were taken from the strain gauge sensor connected to the samples prepared by the hotspot stress method. Using the finite element method, different types of models were analyzed and experimental data were compared with analysis outputs. As a result of the comparison, the most accurate welded joint analysis modeling with the hotspot method has been determined by the results of the experiment and analysis and has been proven with an accuracy rate of 89%.

Fatigue evaluation and structural optimization of rib-to-diaphragm connection in orthotropic steel decks

The fatigue cracking at transverse diaphragm cutouts and rib-to-diaphragm connections is one of the primary ways in which orthotropic steel decks (OSDs) are damaged. In this study, the fatigue damage mechanism of the cutouts and rib-to-diaphragm connections was investigated through a full-scale model experiment and numerical simulations. The applicability and accuracy of different fatigue evaluation methods for the cutouts and rib-to-diaphragm connections were compared. Furthermore, the fatigue behavior of the novel rib-to-diaphragm connection was evaluated, and its optimized structure was suggested based on parametric analysis. The results show that a complex stress distribution dominated by in-plane stresses was observed at the rib-to-diaphragm connection under wheel loading. The cracking of the weld toe at the transverse diaphragm side was found to be the dominant failure mode. The hot spot stress method has a higher prediction accuracy than the critical distance theory for predicting the fatigue life of the cutouts. For the longitudinal rib-to-transverse diaphragm connection, the hot spot stress method could be used to predict the fatigue life of U-rib side weld toes based on fatigue grade FAT 90, and the equivalent structural stress method provided a reasonable prediction of the fatigue life for all fatigue failure modes. Last but not least, a novel rib-to-diaphragm connection structure that can effectively improve fatigue performance was suggested, and its fatigue life was evidenced to be greater than 50 million cycles after geometrical parameter optimization.

Dynamic responses and fatigue assessment of OSD in heavy-haul railway bridges

With increasing freight volume and speed in heavy-haul railways, the impact of train loads on the orthotropic steel deck (OSD) has significantly increased, resulting in fatigue damage of bridges. In this study, the dynamic responses and fatigue assessment of OSD in heavy-haul railway bridges were carried out based on a coupled train-track-bridge analysis and hot spot stress (HSS) method. First, using Dongting Lake Bridge on Hao-Ji Railway as an example, a finite element (FE) model of OSD was established. Then, a simplified method of train loads is proposed to improve the calculation efficiency. The dynamic response results from the coupled train-track-bridge analysis and the moving train load method were compared. The most fatigue-prone detail was determined to be the rib-diaphragm joint weld at the notch. Subsequently, the HSS results of OSD for different scenarios were compared and analyzed. Moreover, the fatigue life of OSD was obtained. Due to multiple stress cycles and large local stresses caused by the heavy-haul train load, the fatigue life of OSD fatigue-prone detail is estimated to be 25.74 years. Finally, the effects of different parameters (e.g., axle weight, train speed, and track irregularity) on the fatigue life of OSD were analyzed. This study provides a reference for the design and evaluation of OSD in heavy-haul railway bridges.

Research on fatigue reliability prediction model and structural improvement of welded drive axle housing based on master S‐N Curve method

AbstractFatigue reliability prediction of welded structures is mainly based on nominal stress or hot spot stress method, but there are some problems such as grid sensitivity and joint geometry dependence. The Master S‐N curve method can solve these problems well, but the corresponding reliability model needs to be studied. In this paper, the fatigue reliability model of welded structures based on the Master S‐N curve method is studied. Considering the randomness of life and the correlation of failure, a reliability model is proposed, which reduces the computational burden by establishing a median damage‐random threshold rule. Taking the welded drive axle housing as an object, the system reliability is analyzed under the bench test condition, and verified by the experimental data. After the verification, this method is used to predict the reliability of the axle housing under variable amplitude loading collected in the test field, and the results are verified by Monte Carlo (MC) method. When the P‐S‐N curves are parallel, the model is accurate, which is the characteristic of the Master S‐N curve method. This method only needs to input the median damage value of the weak part, which is easy to be applied. This method can speed up the reliability prediction cycle of welded structures, which is beneficial to product innovation and optimal design. Finally, an improved design scheme is proposed for the weak parts of welding, and the effects of welding leg width, welding depth, and closed weld on fatigue life are revealed.

Research on the Fatigue Performance of Composite Girders with CSW and ST Truss

To study the fatigue performance of a composite girder with corrugated steel web (CSW) and steel tube (ST) truss, based on an engineering object, a test and finite element (FE) analysis were carried out. The fatigue failure mode and the sensitivities of the hot spot stress to the geometric parameters of the composite girder above were studied. The position with the maximum hot spot stress and the fatigue crack position were discussed and determined. Furthermore, a fatigue life evaluation method was discussed. It was found that linear extrapolation was applicable to finding the hot spot stress. The fatigue performance evolution comprised three stages, including initiation, expansion, and failure of the fatigue crack. The fatigue crack initiation stage accounted for about 95.6% of its fatigue life, and the fatigue crack was opening type I. The maximum hot spot stress emerged at the endpoint of the inclined web of CSW (point S). The main reason for the behavior was that the inclined web had a small out-of-plane flexural stiffness but had to bear an out-of-plane bending moment. Moreover, for the same section of the CSW, the hot spot stress on the molding side was less than that on the non-molding side. The hot spot stress exhibited negative correlations with the chord diameter, chord thickness, and bend radius, meanwhile, it had the highest sensitivity to the chord diameter. The existing S-N curves were not suitable to evaluate the fatigue life of the composite girder with CSW and ST truss. Since the intersecting weld between the CSW and the chord was located in longitudinal welded joints, the hot spot stress method suggested adopting the basic fatigue strength of 100 MPa, corresponding to 2,000,000 load cycles. Meanwhile, an empirical calculation model of the S-N curve for the composite girder with CSW and ST truss was created.